Radar technology for object detection and ranging is well established in high-end military applications but has recently had increasing impact in commercial and industrial equipment. The design of multi-channel radar systems used in combination with phased array transmitters and beamforming signal processing enable resolution of a large number of targets at different angles, even if the targets are located at the same range. The design of the antenna system in such a configuration, particularly for three-dimensional measurement applications that require the radar unit to scan the field of view in the horizontal as well as the vertical dimension, is challenging.
Low-cost applications commonly apply planar patch antenna arrays, such as two-dimensional (2D) uniform planar arrays or circular configurations. The antenna elements (or patches), which are placed in a certain uniform or nonuniform structure in two dimensions, radiate and/or receive the electromagnetic (EM) signal generated by the radar frontend to freespace with a certain polarization of the EM wave. Broadband radar systems, which use a large EM signal bandwidth to achieve high accuracy, commonly use so-called linear polarized antenna elements, with the polarization direction depending on the patch type, feeding point, and angular rotation of the patch on the substrate material. These antennas are commonly fabricated on multi-layer printed circuit boards (PCBs).
For proper operation of the radar unit, it typically is required that the polarization direction of all antenna elements, transmitters as well as receivers, is geometrically aligned, since a mismatch of the polarization plane results in signal loss and therefore reduced system performance. Circular polarized antenna elements, which are not sensitive on the rotation angle, often cannot be used since the bandwidth of this latter antenna type is very limited.
As a second problem, depending on the array type, the single antenna elements have to be closely spaced, e.g. by half a wavelength of the transmitted EM signal in conventional uniform rectangular planar arrays.
Thus, there are many challenges in designing planar arrays, including placing the antenna elements in closely spaced positions; aligning the antennas to achieve similar EM field polarization of the individual patches; and feeding the antenna elements from the radio frequency (RF) frontend, avoiding feed line intersections. Contradictions appear in trying to meet all of these requirements simultaneously and often strictly limit the practically realizable configurations for array design.
A conventional solution to the above challenges applies a so-called “backside”-feed to the patch elements. In this case, the antenna elements, which are located on the top layer of the antenna PCB, are fed from the back, either from a waveguide feeding structure or a conventional buried transmission line based approach on an inner PCB layer. Waveguide feeds, although being optimum regarding losses, are bulky and very expensive in fabrication. Buried transmission lines, on the other hand, are inexpensive to fabricate but introduce higher losses especially at high operating frequencies (without a separate layer of expensive substrate materials).
If feeding lines on the PCB's top layer are used, the orientation of the patches and the location of the feeding point must match. One solution to achieve this in a closely spaced two-dimensional L-shaped array is to use a 45-degree orientation of the patches. This in turn reduces the spacing of the antenna elements and therefore leads to stronger coupling effects, which is disadvantageous for the overall array performance.
Therefore, there is a need for improved radar antenna arrays.